<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        [v-cloak] {
            display: none;
        }
        
        .a1 {
            color: red;
        }
        
        .a2 {
            font-size: 50px;
        }
        
        .a3 {
            background: #dbdbdb;
        }
    </style>
</head>

<body>
    <div id="main" v-cloak>
        <!-- <h1 class="a1 a2 a3">{{msg}}</h1> -->
        <!-- <h1 :class="['a1','a2',{'a3':flag}]">{{msg}}</h1> -->
        <h1 :class="{a1:false}">{{msg}}</h1>
        <h1 :style="{color:'red'}">{{msg}}</h1>
    </div>


    <script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
    <script>
        var vm = new Vue({
            el: '#main',
            data: {
                msg: 'Hello Vue1',
                flag: false
            }
        })
    </script>

</body>

</html>